Ionization-type fire or smoke sensing system

ABSTRACT

To render an ionization-type sensing element essentially immune to changes in ambient conditions, principally temperature or operating voltage, while still using low-current draining circuits to sense response of the ionization chamber of the sensor, the threshold response level of a field effect transistor (FET) is arranged to have approximately the same temperature response characteristic, within the range of ambient temperature considered, as the ionization cell so that the overall circuit or system combination of the cell and FET amplifier will have a response which is essentially independent of ambient temperature or similar conditions. The source path of the FET is connected to a voltage divider which is so dimensioned that the voltage division ratio (R 2  /R 1 ) is related to the temperature coefficient (α) of the base-emitter voltage of the FET and the temperature coefficient (β) of the measuring ionization cell chamber: 
     
         R.sub.2 /R.sub.1 =(β/α)-1.

Cross reference to related patents, all assigned to the assignee of thepresent application: U.S. Pat. Nos. 3,710,110; 3,767,917; 3,908,957.

The present invention relates to an ionization-type fire and smokesensor having a sensing element which includes a measuring ionizationchamber connected in series with a resistor and a field effecttransistor (FET) connected to the sensing element. The field effecttransistor provides an amplified output; its characteristics are soselected with respect to the measuring ionization chamber and theoverall circuit that the FET becomes conductive when the sensingionization chamber provides an output voltage in excess of apredetermined threshold value. Reference is made to the cross-referencedpatents for construction of such sensors.

Ionization-type smoke and/or fire sensors must meet high and difficultrequirements: Operating as smoke detectors, they should provide asensing indication as early as possible upon occurrence of a fire; theymust, however, also operate under severe ambient conditions, and shouldbe essentially immune to climatic influences such as temperaturechanges, wind, humidity, presence of corrosive gases, overall corrosion,and should additionally be immune to electrical extraneous influences,such as changes of supply voltage. Further, the sensors, when combinedin a fire alarm system, must operate economically, that is, with lowquiescent current, so that many sensing units can be located in thespace to be supervised, or the supervised space extended. Additionally,the operating condition and operability of the sensors to providesensing output should be capable of being checked simply by electricaltest circuits.

It is difficult to provide an ionization-type sensor which desirablymeets all the requirements placed thereon; known sensors, while highlysatisfactory in many respects, can still be improved to provide forbetter operating and application characteristics.

It has previously been proposed to make ionization-type smoke detectorsessentially independent of operating voltage by using a saturatedreference ionization chamber as a resistance element in the circuitsystem. Change of the operating voltage then holds the voltage appliedto the measuring ionization chamber, that is, the unsaturated chamber,at a constant level. Such systems have the disadvantage of hightemperature dependence, that is, the temperature response characteristicof the measuring ionization chamber varies greatly with temperature. Thealarm threshold output level of the measuring ionization chamber shiftsapproximately linearly with temperature. Use of symmetrical ionizationchambers in which ambient temperature changes mutually compensate eachother have been proposed; the alarm threshold level of the fire or smokesensor still is dependent on operating voltage, however.

Smoke and fire sensing elements which are included in a sensing andalarm system should use as little current as possible. The elementsshould be economical in operation. It has been proposed to use an FET asthe first electronic active element of the evaluation circuitryconnected to the ionization chamber, for example by connecting thecontrol or gate electrode of the FET with the junction between themeasuring ionization chamber and the reference ionization chamber. TheFET is normally in the blocked state. The source voltage is so connectedand dimensioned that it is higher than the blocking voltage. Suchionization-type smoke detectors have the disadvantage that the alarmthreshold level then changes with changes in ambient conditions.Simultaneous compensation for all changes in ambient conditions was notpossible since the customary arrangements to effect such compensationwere usually mutually exclusive.

It is an object of the present invention to provide an ionization-typesmoke and fire detector system which, simultaneously, meets thefollowing requirements.

(1) Economy; (2) independence of the alarm threshold of operatingvoltage; (3) independence of the alarm threshold of ambient surroundingtemperature-- within reasonable limits; (4) low quiescent current, (5)simple checking or test arrangements by electrical checking or testing.

Subject matter of the present invention

Briefly, the fire or smoke sensing system includes an arrangement toautomatically control the threshold value, within suitable limits ofapplication, which has a substantially similar temperature responsecharacteristics as that of the sensing element, and which, by itsinherent operation, retains the threshold value of response, within thetemperature range, essentially independent of ambient temperature.

The system, in a preferred form, includes an FET which is connected tothe ionization chamber, and is further connected to a control circuit.[.which has a temperature response characteristic which maintains thethreshold of response of the FET essentially independent of ambienttemperature.].. In accordance with a preferred embodiment, a voltagedivider is connected to the FET, the tap point of the voltage dividerbeing connected to the gate or control electrode of the FET, and thevoltage divider being so dimensioned and arranged relative to the FETand the ionization chamber to render the overall response essentiallytemperature independent.

The invention will be described by way of example with reference to theaccompanying drawing, wherein the single FIGURE is a schematic circuitdiagram of the system of the present invention.

The ionization smoke detector cell is an unsaturated sensing ionizationchamber MK. This chamber MK is exposed to ambient air as schematicallyindicated by the broken lines thereof. The ion current within thischamber is dependent on smoke concentration in the air to which thechamber is exposed. Chamber MK is connected in series with a referenceionization chamber RK. Reference ionization chamber RK is essentiallyclosed and saturated. The junction point of the two ionization chambersMK and RK is connected to the gate electrode of an FET. The FET is, forexample, an MOS-FET, preferably with a high gate resistance. A typicalFET useful in the present invention is of the type MEM 520 (GeneralInstruments). The source path of the FET is connected to an electricalcircuit including the collector-emitter path of a transistor T1 andresistors R2 and R1 connected in parallel thereto and forming a voltagedivider. The tap point of the voltage divider is connected to the baseof the transistor T1. The resistors R1, R2, or at least one of thempreferably, are adjustable.

Operation: The source voltage U_(S) for the FET is determined by thecircuit formed by transistor T1 and the resistors R1, R2. This sourcevoltageU_(S) is so selected that the sum of the voltage U_(S) and thethreshold voltage of the FET is slightly greater than the voltage dropU_(K) across the measuring ionization chamber MK when the ionizationchamber is in quiescent, that is, non-smoke sensing condition. Thethreshold voltage of chamber MK, therefore, when smoke or fire aerosolsare absent, will be slightly greater than the threshold voltage of theFET and hold the FET in blocked, non-conductive condition. If smoke orfire aerosols penetrate into the measuring chamber MK, the resistancethereof will increase and, as soon as the voltage drop U_(K) exceeds thesum of the voltage formed by the source voltage and the thresholdvoltage, the FET will become conductive and an alarm current will flowover the lines U₁ and U₂ to a central alarm station (not shown).

The drain circuit of the FET may include a further resistor; voltagedrop across that resistor may, as known, control other switchingdevices, for example additional alarm circuits.

The voltage drop U_(K) across the measuring ionization chamber of theionization sensor is highly dependent on ambient temperature. Thus, whensuch an ionization chamber is used in practical environments, the alarmthreshold level will change in accordance with ambient temperaturechanges. Such an ionization chamber, thus, would respond later with sometemperatures than with others. In order to avoid this highly undesirablecharacteristic of the ionization chamber, the electrical circuit is soarranged that the temperature coefficients of the circuit in series withthe FET are similar to that of the measuring ionization chamber MK.Accordingly, the temperature coefficient is obtained by selecting therelationship of the resistors R₂ /R₁, and hence the amplification of thetransistor T1 in such a way that the difference of U_(K) and U_(S) willremain constant upon changes in temperature. This means, of course, thatthe resistance relationship must be matched to the temperaturecoefficient of the transistor T1. The resistors R1, R2 may, if desired,also be temperature-responsive resistors to further enhance the effectsof the circuit, that is, the resistors may be temperature dependent, andso arranged that the above referred-to condition will be fulfilled, thatis, that (U_(K) - U_(S)) will be independent of temperature, at leastwithin a certain temperature range which is usual in the space where theionization sensor is employed, for example within the temperature rangethrough which the ambient temperature varies. If the ionization chamberis to be employed under extreme conditions, the range should be selectedto be approximately in the area of normal or most applicable operatingtemperature. Various operating conditions can be matched by adjustmentof one, or both of the resistors R1, R2.

In an actual example, an ionization chamber which has a chamberstructure MK as disclosed in cross-referenced U.S. Pat. No. 3,908,957was combined with a system in accordance with the present invention toprovide an approxiately constant temperature response. The temperatureresponse characteristic of this chamber, by and itself, is essentiallylinear in the temperature range between -10° C. and +50° C. Thetemperature coefficient is:

    U.sub.K =U'.sub.k +βΔT

in which U'_(k) is a base constant, and β is -25 mV/°K.; β thus isexpressed in mV/°K. Transistor T1 is a silicon transistor having atemperature-vs.-base emitter voltage characteristics of; U_(BE) =U'_(BE)+αΔT, in which the temperature coefficient is: α=-1.5 mV/°K. Thecollector-emitter voltage, and thus the source voltage of the FET, willthen be: ##EQU1## Substituting in the relationship (U_(K)-U_(S))=constant then results in a resistance ratio for the voltagedivider: R₂ /R₁ =(β/α)-1. This ratio will then provide for an alarmthreshold which is constant and temperature independent within the rangeof linear temperature-resistance characteristics of the chamber MK andof the transistor T1. The resistance ratio for the above ionizationchamber and transistor will then be: R₂ /R₁ =15.6. At this resistanceratio, the circuit will be temperature independent. In a practicalexample, transistor T1 was of the type BC 320, resistor R1=10 kΩ, andresistor R2= 150 KΩ. The FET was of the type MEM 520, having a thresholdvoltage of about 3.5 V. The line voltages U₁ and U₂ were about 20 V. Thechamber voltage UK', under non-conductive condition, was about 8 V. Thissystem then is essentially temperature independent within a widetemperature range, and operates with improved and uniform sensitivitythroughout that range while being, additionally, essentially independentof operating voltage. This arrangement also has the other advantagesrequired, in that the quiescent current is extremely small, while beingmade of components which are simple and inexpensive, so that the overallsystem can be cheaply made.

The system has a further advantage. It is a simple matter to superviseoperability thereof. Introducing an additional resistor R3 in serieswith the measuring chamber MK and connecting a control line to a testterminal U₃ permits checking of the operability of the chamber. Theresistor R3 may, for example, have about 20 kΩ.

Various changes and modifications may be made within the scope of theinventive concept.

We claim:
 1. Ionization-type fire or smoke sensing system havinga sourceof electrical supply (U1, U2): a measuring ionization chamber (MK); aseries resistance element (RK) connected in series with the measuringionization chamber (MK); a field effect transistor (FET) connected tothe measuring ionization chamber, said FET having a conduction thresholdvoltage which is just above the level of the output voltage (U_(k)) ofthe measuring ionization chamber (MK) when smoke or fire aerosols areabsent, so that, upon presence of smoke or fire aerosols, the outputvoltage of the chamber will rise and the FET will become conductive toprovide an output signal; and temperature control means (T1, R1, R2) torender the circuit combination of the ionization chamber (MK) and theFET essentially independent of temperature within a given range,comprising a transistor (T1) having its collector-emitter path connectedin the source supply path of the FET; and a voltage divider (R1, R2)connected in parallel with the collector-emitter path of the transistor(T1), the tap point of the voltage divider being connected to the baseof the transistor (T1); to provide a temperature compensationcharacteristic which has the same relative control direction as thetemperature characteristic of the measuring ionization chamber (MK) anddimensioned with respect to the FET to maintain .[.the conductionthreshold thereof.]. .Iadd.the difference of said output voltage (U_(k))and of the voltage (U_(s)) at the source of said FET.Iaddend.essentially independent of temperature within said range. 2.System according to claim 1, wherein at least one of the resistors ofthe voltage divider (R1, R2) is adjustable.
 3. System according to claim1, wherein at least one of the resistors of the voltage divider (R1, R2)is a temperature dependent resistor having a resistance value whichdepends on ambient temperature.
 4. System according to claim 1, furthercomprising a further resistor (R3) connected in series with themeasuring ionization chamber (MK) and the source voltage;the junctionpoint between the further resistor (R3) and the chamber (MK) forming atest voltage terminal (U3).
 5. System according to claim 1, wherein theseries resistance element (RK) comprises a saturated referenceionization chamber.
 6. System according to claim 1, wherein thetransistor (T1) and the voltage divider (R1, R2) are selected to have atemperature characteristic such that the voltage drop of the networkformed by the voltage divider (R1, R2) and the transistor (T1) hasessentially the same temperature coefficient as the temperaturecoefficient of the voltage drop across the measuring ionization chamber(MK), so that the temperature characteristics of the source voltageapplied to the FET will be similar to the temperature characteristics ofthe voltage drop across the measuring ionization chamber.
 7. Systemaccording to claim 6, wherein the voltage division ratio (R₂ /R₁), thetemperature coefficient (α) of the base-emitter voltage of thetransistor (T1), and the temperature coefficient (β) of the measuringionization chamber (MK) have at least approximately the followingrelationship:

    R.sub.2 /R.sub.1 =(β/α)-1,

wherein R₂ /R₁ is the ratio of resistance values of the voltage divider;α is the temperature coefficient of the base-emitter voltage of thetransistor (T1) and β is the temperature coefficient of the measuringionization chamber (MK).
 8. System according to claim 7, wherein the FETcomprises a high gate resistance, MOS-type FET.
 9. System according toclaim 7, wherein the series resistance element (RK) comprises asaturated reference ionization chamber and wherein the FET is of the MOStype.